Sol-gel techniques are commonly used to produce dense thin films in semiconductor fabrication. The word sol-gel, however, does not describe a product but a reaction mechanism whereby a sol may be transformed into a gel. A sol is a colloidal suspension of solid particles in a liquid; one method of forming a sol is through hydrolysis and condensation reactions which cause a multifunctional monomer in a solution to polymerize into relatively large, highly branched particles.
Many monomers suitable for polymerization are metal alkoxides. For example, a tetraethylorthosilicate (TEOS) monomer may be partially hydrolyzed in water by the reaction EQU Si(OEt).sub.4 +H.sub.2 O.fwdarw.HO--Si(OEt).sub.3 +EtOH
Reaction conditions may be controlled such that, on the average, each monomer undergoes a desired number of hydrolysis reactions to partially or fully hydrolyze the monomer; TEOS which has been fully hydrolyzed becomes Si(OH).sub.4. Once a molecule has been at least partially hydrolyzed, two molecules can then link together in a condensation reaction, such as EQU (OEt).sub.3 Si--OH+HO--Si(OH).sub.3 .fwdarw.(OEt).sub.3 Si--O--Si(OH).sub.3 +H.sub.2 O
or EQU (OEt).sub.3 Si--OEt+HO--Si(OEt).sub.3 .fwdarw.(OEt).sub.3 Si--O--Si(OEt).sub.3 +EtOH
to form an oligomer and liberate a molecule of water or ethanol. The Si--O--Si configuration in the oligomer formed by these reactions has three sites available at each end for further hydrolysis and condensation; thus additional monomers or oligomers can be added to this molecule in a somewhat random fashion to create a highly branched polymeric molecule from literally thousands of monomers.
Through continued reactions, one molecule in the sol may eventually reach macroscopic dimensions so that it forms a network which extends throughout the sol; at this point (called the gel point), the substance is said to be a gel. By this definition, a gel is a substance that contains a continuous solid skeleton enclosing a continuous liquid phase. As the skeleton is porous, a gel can also be described as an open-pored solid structure enclosing a pore fluid. An oligomerized metal alkoxide, as defined herein, comprises molecules formed from at least two alkoxide monomers, but does not comprise a gel.
An ungelled sol may be dip-coated or spin-coated onto a substrate to form a thin film on the order of several microns or less in thickness, gelled, and dried. In practice, such a thin film is subjected to rapid evaporation of volatile components, to the extent that the deposition, gelation, and drying phases may all be taking place at once as the film collapses rapidly to a dense film. Drying by evaporation of the pore fluid produces extreme capillary pressure in the microscopic pores of the wet gel, causing many pores to collapse and the gel to be reduced in volume as it dries, typically by an order of magnitude or more.
A dried gel which is formed by collapsing and densifying a wet gel during drying is termed a xerogel. A thin film xerogel is usually dense, with just a few percent porosity remaining after drying. U.S. patent application Ser. No. 08/247,195 to Gnade, Cho and Smith discloses a process for producing an aerogel thin film on a semiconductor substrate; an aerogel thin film is distinguishable from a xerogel thin film primarily by a manner of drying which largely avoids pore collapse during drying of the wet gel; this results in a substantially undensified thin film which can be fabricated with almost any desired porosity (thin films with greater than 90% porosity have been demonstrated). Such films have been found to be desirable for a low dielectric constant insulation layer in microelectronic applications.